Celestia Forums

Hey, a thought just occoured, and Im wondering if anyone could help me out. Ive been thinking about the idea of space parasols, a technique that would be used to terraform Venus for instance. The main problem with them seems to be stability - they have trouble maintaining their position in space due to light pressure from the sun. This can be counteracted by building slightly more sunward from a planets L1 point, or by controlling position with boosters and such. However, I was wondering just how much of an effect colour would have on the light pressure effect?
Since white reflects nearly all incoming radiation, while black absorbs most of it, would it be easier to create a black solar parasol than a white one?? Just how much a difference would it make? Also, would a darker parasol get too hot?

If you build the parasol slightly sunward of the L1 point the parasol will not be in the correct orbit, so will drift off station. No matter what you do the shade will drift, so you will need some kind of station keeping.

I think that colouring the shade black won't help. as the momentum of the light is transferred whether the photons are reflected or absorbed. (please correct me if I am wrong).

My suggestion is to make the shield like a drogue parachute, with a counterweight on the sunward side; a system of controllable guy ropes and flaps would allow the drogue to maintain constant active control. This action could be powered by solar energy collected by the shade itself.

Alternately the parasol could be a swarm of inflatable spheres or disks each under active control, a nightmare to coordinate without using some sort of smart AI program.

This page I have made shows a sunshade created in Celestia (near the bottom of the page); Grant Hutchinson wrote the .ssc for this parasol, and I have used Runar Thorvaldsen's texture for a terraformed Venus.
http://www.orionsarm.com/worlds/Venus.html

I think that colouring the shade black won't help. as the momentum of the light is transferred whether the photons are reflected or absorbed. (please correct me if I am wrong).

Remember that momentum is conserved. If the photons were reflected, twice as much momentum will have been transferred to the parasol as if the photons were absorbed.

Good; that means that a black parasol would not be accelerated by light pressure as much.
Of course the black parasol would heat up more, but that would just lead to omnidirectional emission of infrared (that isn't the word I am looking for; perhaps isotropic is more correct).

There also will be unexpected accelerations due to the solar wind as well as CME events and asteroid collisions. Some kind of active station keeping would be necessary, I'm sure. One suggestion I've seen would involve high power lasers, with their light beams pushing the parasol around. The lasers themselves might be separate satellites or even ground based if the parasol (or reflector) is for the Earth or Mars.

Remember that the material making up the parasol has to be incredibly flimsy -- perhaps only a few molecules thick. Otherwise an absurd amount of material would have to be used.

If it was that filmsy it would have to be spun in order to avoid just collapsing in on itself wouldnt it? Well unless you took the resources to build some sort of rigid framework... but that would be even heavier than the canvas itself.
I thought you would still need station keeping equipment with a black parasol, the idea was that you would need far less of it if light pressure wasnt such a problem, thus cutting down on resources and energy needed As you say its going to need a hell of a lot of resources anyway, so any corners you could cut would be worth it.... heh.

Neethis wrote:If it was that filmsy it would have to be spun in order to avoid just collapsing in on itself wouldnt it?

yes, indeed.